Delaying tactics

Have you ever wondered why you cannot hear the announcements at train
station? It is all to do with time delays.

Dozens of speakers scattered around the station deliver the same announcement
at exactly the same time. If you can hear several speakers that are different
distances from you, the sounds arrive at your ears at slightly different times
and the message becomes garbled.

If an announcement is intelligible, it is usually because you are so close to
one speaker it drowns out the others. Alternatively, you could be exactly the
same distance from the two closest speakers, so their sounds arrive at the same
time and are reinforced at your position, just as they would be in a stereo
system.

Time delays can also garble the sound of a multi-channel home-theatre system.
In most rooms, the rear-channel speakers will be further from the listening
position than the front speakers, while the centre-channel speaker will be
closer than any other speaker in the room. If you do not compensate for the
different distances, the purity of the sound will be affected. It will never be
as bad as a platform announcement because the distances involved are too short,
but there will be a small effect.

Good-quality receivers, such as Panasonic's new SA-HE90, provide special
circuits that delay the sound sent to the speakers closest to you so that the
sound from all speakers arrives at exactly the same time. Panasonic's circuitry
is simpler than most, not allowing adjustments for individual channels, but it
is sufficient for most situations.

The SA-HE90 also allows compensation for volume level. Here, the idea is not
to compensate for differences in volume caused by distance, but from using less
efficient speakers in the surround channels.

Most main speakers have an efficiency close to 90dBSPL. Surround speakers are
often 87dBSPL-efficient or less. This means you have to boost the volume of the
surround speakers by +3dB to balance the sound field. A test signal generated by
the Panasonic lets you balance speaker levels either by careful listening or
with the aid of a sound-level meter.

Other useful circuits inside the Panasonic that require precise adjustment to
guarantee the best sound are those controlling speaker size selection, subwoofer
cut-off frequency and dynamic-range compression, centre width and dimension.

The SA-HE90's instruction manual is not easy to follow because it covers two
different models with different controls, one of which has two variants. This
makes setting up and correctly calibrating the receiver more difficult than it
should be.

If you are unfamiliar with home-theatre receivers, or short-tempered, we
would suggest asking the salesperson for a quick lesson.

When set up correctly, the Panasonic performs well. The output stage uses
full-size MOSFET devices (rather than the monolithic ICs used in Panasonic's
cheaper models), which deliver 60 watts per channel into eight-ohm loads under
most conditions. The Dolby Digital and ProLogic II decoders ensure smooth, clean
surround sound from DVD.